Colour causes a depth illusion in human visual perception

Abstract UNING FUNCTIONS). REPLICATED. EXTENDED KINGDOM 2003.

SUCCESSFUL REPLICATION. FOUND THAT SOME THINGS CHANGED WITH

ORIENTATION / SF/ PHASE

Introduction Scientists are interested in how colour sensitive and luminance sensitive mechanisms interact

when a subject is presented with stimuli that embody the particular relationships that exist

between colour and luminance in the human vision system.

Colours plays a highly crucial role in the vision characteristics of the human sight, because of

its immense sensitivity the subject holds great importance among vision scientists throughout

the globe (e.g. review by Regan, 2000). The main strategy to gain knowledge in this subject

matter is to study the performance, set against a certain criteria, using only iso-luminant

(single colour characteristics) and iso-chromatic (multiple colour characteristics) stimuli. The

In this study, we aim to study the colour and luminance characteristics of the human vision

with an attempt to establish a proper relationship between the colour, depth and luminance in

the human vision. Kingdom (2003) proposed that a significant amount of knowledge could be

gain by analysing the behaviour of colour and luminance in the human vision perception, and

how this phenomenon can emulate the spatio-temporal relationships between the colour and

luminance found in human vision. Kingdom et al (2005) attempted the first successful

approach to understand this subject and investigate the proper relationship in such

phenomenon. They studied that when a chromatic grating is added at a certain level to

luminance grating; one of them gains the impression of groovy structure, this process of

transference from colour to shape is called the Depth Enhancement. On the contrary, if

second grating of chromatic grating is further added to this process at a different level, the

impression of depth is either reduces or completely eliminated, and this process of

elimination or reduction is called Depth Suppression. These kinds of phenomena are

generally experienced in achromatic kinds of studies that are highly influenced by different

colour contrasts (Lehky & Sejnowski, 1988; Ramchandran, 1988; Attick et al., 1996; Sun &

Perona, 1997).

The depth enhancing processes, formation of a shape from shade due to the grating between

chromatic and luminance patterns proposed that natural human visual system has certain

inbuilt capabilities;

1. The main cause of variation in chromatic and luminance behaviour that are spatially

aligned against each other is due to the variation in surface reflectance.

2. The main cause of pure or impure variations in the luminance behaviour is due to the

non-uniform illumination, such as shading and shadows.

These physical relationships between the chromatic and luminance grating holds

dominant importance in the field of human vision and the scientist are contented upon

the agreement that such relationships give rise to acknowledge these in-built system in

the human vision (Rubin & Richards, 1982; Cavanagh, 1991; Mullen & Kingdom,

1991; Olmos & Kingdom, 2004) along with the colour shading affect which is quite

evident to appreciate that these are embedded into the human visionary configuration.

There are several other factors that may signal the perceptions of surface shapes. There is

interest in whether colour contrast on the perception of shapes influences the perceptions of

shading, such as texture; and whether the colour contrast influence the contribution of

shading to surface curvature when it is present alongside other cues. It is possible that the

influence of colour contrast on shape-from-shading is reduced, or even eliminated, when

surface information other than colour is present, because in such circumstances the surface

versus illumination interpretative role of colour contrast becomes redundant. The aim of this

study, as we have already mentioned, is to establish a relationship between the colour,

luminance and depth of human vision with a focus to investigate the influences of the colour

contrast on perceived shapes in pattern that produces shape from shading with shape from

texture. Mamassian and Landy (2001) also noticed that the orientation defined textures have

been shown to combine synergistically with shading to create strong impressions of depth.

Numerous questions have so far been aroused concerning the chromatic properties of the

colour shading. It is often bring into consideration to investigate that the combination of two

phenomena, depth enhancement or depth suppression, in colour directions is more important.

There is high possibility the colour shading effect is weaker when the directions of depth

enhancement and depth suppression phenomena is same, as in such phenomenon the human

vision system might bound together both colouring patterns into a single object, releasing the

luminance variations from being designated as changes in reflectance, and designating them

instead as shading, even though they are spatially aligned with one of colour patterns.

In this study, we have attempted to answer the questions regarding mixed colour and

luminance plaids with an aim to manipulate the direction and colour texturing of both the

depth enhancement and depth suppression. The results of this study furnishes further about

the information and understanding of the chromatic properties in terms of colour shading

textures and formation of shape from shading by the natural human vision system, and

therefore tries to acknowledge the assumptions related to the relationships between the

colour, luminance and depth of the vision system. In order to grab proper and precise

knowledge about the relationship between the colour, luminance and depth in the perception

of vision, we have utilize an adjustable stimulus that have original and real corrugations and

bumps in its structure, defined stereoscopically.

The findings of this study can be summarised by suggesting that the impression of depth is

presented when variations in colour were appeared at different orientation to plaid gratings

and at the same orientation but out of phase, therefore, the colour variations at different

orientation and out of phase will yield the depth enhancement. Additionally, the addition of

colour variations of the same orientation and in phase will suppress the grating that will yield

the depth suppression.

Method

Participants

The participants who took part in the chromatic-achromatic experiment were 8 psychology

students and 1 professor in the University of York: A, E, S, J, R, JS, JR, H and AW. For the

other 5 experiments, there were 8 participants in total except subject A. Details such as age,

gender and handedness were not necessarily collected for this experiment. Personal

identifying information were used anonymous.

Materials

The materials used in this experiment were stimuli viewed though a CRT screen and a

keyboard. First of all, the screen was a NEC Multisync 200 screen and the diagonal size of

screen was 20 inch. The refresh rates of how quickly the screen updates were 100Hz. Figure

1 shows the viewing distance between eyes and the screen. The viewing distance was the

total distance (1+2+3+4+5) about 700mm, but not the straight distance between eyes and

screen. The field of view (w x h) was 29.50 x 22.34 deg. The structure of depth (disparity)

was the distance between fovea and the place of images. Each eye was separated to see the

stimuli, it was the way that how stereo images achieved. The reason for that was to make

each eye construct different figures to achieve the disparity not to change the overall disparity

of objects.

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Figure 1. Lab settings in this experiment

Secondly, participants’ adjustments of the amplitude of the depth corrugations in the stereo-

images were made by pressing the up and down arrow keys on a standard keyboard. The

mean starting value of adjustments were -6.00 and the range of that was between -5.00 and

+5.00. Subjects’ responses were accompanied with auditory indicators.

Thirdly, stimuli were displayed on a grey background. The perform calibration of all

phosphors for chromatic data showed the location in colour space for 3 guns. It was

Screen

Eyes

Mirror

calibrated by using the Spyder 3 colorimeter. The R (red), G (green) and B (blue) gun outputs

were gamma-corrected after calibration. The CIE coordinates of the monitors’ phosphors

were R: x=0.640, y=0.330; G: x=0.300, y=0.600; B: x=0.150, y=0.060. The stimuli were

constructed from three component gratings: luminance modulated gratings, colour modulated

gratings and drift modulated gratings. All achromatic gratings with contrast of 100% had a

spatial frequency of 2 cpd and an orientation of 90 deg. The stimuli were presented in a

circular, hard-edged window. The achromatic gratings were ‘black and white’, and were

produced by modulating all three RGB phosphors in (1,1,1). The colour gratings were ‘red-

green’, and was designed to dissociate the post-receptor chromatic mechanism that

differences the L (long-wavelength-sensitive) and M (middle-wavelength-sensitive) cones.

The colour space of LMS was (1, -1,0). The drift gratings with contrast of 10% had a spatial

frequency of 1 cpd and were alternated to avoid movement after-effects. Alternation had a

temporal frequency of 1Hz. In this condition, horizontal gratings could not be used because

the shift bars from left to right was not seems to be moved. So, vertical gratings were made to

get larger disparity. All gratings were formed from sinusoidal modulations of cone contrast.

In more details, stimuli were made and divided into 6 conditions in this experiment: 1.

chromatic and non-chromatic, 2. in phase (0°) and out of phase (90°), 3. Orientation (0°, 30°,

60°, 90°), 4. Phase (0°, 30°,60°,90°), 5. Spatial frequency (1, 2, 4, 8 x original), and 6.

Drifting and static.

Design

The IVs (Independent Variables) in this experiment were 6 stimuli conditions. The DVs

(Dependent Variables) was the subjects’ adjustments for each condition. The main design

was a between subjects design.

Procedure

The type of procedure used here was called ‘psychophysics. Participants were asked to

estimate the apparent depth of the corrugations in the stereo-gratings on a CRT screen and

adjusted the amplitude of the depth corrugations in the stereo-gratings until they matched the

apparent depth of the corrugations in the test stimuli by pressing the up and down arrow keys

on a keyboard. There was no time limit. Each testing session took approximately 1 hour and

there were 6 individual conditions. During each session, stimuli were presented in a random

order with several practice trails and test trails. In the experiment 1, 2 and 5, participants were

tested 8 test trails for each condition. In the experiment 3 and 4, participants were tested 5

trails for each condition. Some participants experienced fading of images or other possible

adverse effects such as headache or dry eyes during along time of staring at a computer

screen. So, participants were encouraged to let their eyes roam around the stimuli to avoid the

negative influence on the adjustments. Finally, written consent forms were obtained from all

participants.

Results As illustrated in figure 2, average disparity threshold showed that the chromatic condition

(M= 5.11, SD= 3.32) tends to be higher than the achromatic condition (M=2.93, SD= 2.46).

The mean difference between two conditions was 2.17 and the 95% confidence interval for

the estimated population mean difference is between 0.44 and 3.91. A paired sample test was

carried out to show that the difference between conditions was significant (t= 2.885, df= 8,

p< 0.020, 2-tailed).

Figure 2. Mean disparity threshold for chromatic and achromatic stimuli.

As illustrated in figure 3, average disparity threshold of the in phase stimuli (M= 2.90, SD=

2.14) was lower than the out of phase stimuli (M= 4.91, SD= 3.60). The mean difference

between two conditions was -2.02 and the 95% confidence interval for the estimated

population mean difference is between -4.96 and 0.93. A paired sample test showed that the

difference between conditions was non-significant (t= -1.62, df= 7, p= 0.149, 2-tailed).

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Chromatic Achromatic

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Mean Disparity Threshold

In Phase

Out Of Phase

Figure 3. Mean disparity threshold for in phase and out of phase. Figure 4 showed the

average disparity threshold for four separated orientations: 0 (M= 4.37, SD= 2.88), 30 (M=

3.74, SD= 2.17), 60 (M= 4.08, SD= 2.45) and 90 (M= 2.23, SD= 2.14) degrees. There was a

significant effect (ANOVA?) of the degree of orientation, F(3,21) = 3.207, p= 0.044. Then a

pairwise comparison was carried out to show the difference between each individual degree

of orientation. It indicated that there was no significant difference between 0, 30, 60 and 90

degrees.

Figure 4. Mean disparity threshold for four different degrees of orientation.

Figure 5 showed the average disparity threshold for four individual phase variables: 0 degree

(M= 2.34, SD= 1.69), 30 degree (M= 3.26, SD= 2.16), 60 degree (M= 4.48, SD= 2.72) and

90 degree (M= 4.46, SD= 2.67). A non-significant effect of phase shift was found, F (3,21)=

2.997, p= 0.054. Then a pairwise comparison was conducted which indicated that four

individual phase degrees were not significantly differ from one another.

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0 30 60 90

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Orientation (degrees)

Figure 5. Mean disparity threshold for four different degrees of phase shift. As illustrated in

figure 6, average disparity threshold for spatial frequency of 1(M= 4.48, SD= 2.26), 2 (M=

3.00, SD= 1.86), 4 (M= 2.56, SD= 1.91) and 8 (M= 2.48, SD= 1.89) showed a significant

effects, F (3, 21) = 5.232, p= 0.007. After which, pairwise comparisons was employed and a

significant difference was found between spatial frequency 1 and spatial frequency 2 (p=

0.042). There were no significant differences between the spatial frequency 1 and 3, 1 and 4

and 3 and 4.

Figure 6. Mean disparity threshold for four types of spatial frequency.

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Phase shift (degrees)

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Spatial Frequency

Figure 7 indicated the average disparity threshold for drifting (M= 3.77, SD= 3.06) and static

(M= 2.04, SD= 1.64) images. There was a non-significant effect (t= 1.75, df= 7, p= 0.124, 2-

tailed) between these two conditions by using a paired sample test.

Figure 7: Mean disparity threshold for drifting and static stimuli.

Discussion The present study for the Colour causes a depth illusion in human visual perception describes

based on the number of experiments and detailed analysis as done in the study that color

sensitive and luminance sensitive mechanism can be observed from it. The results of the

above study can be summarised based on the analysis as follows:

Based on the capacity to enhance and suppress depth as detected by the interference in the

mixed color plus luminance plaids it can be observed that the L – M and S gratings during

interference are similar. The observed capacity of the mentioned grating as above for getting

the desired depth does not depend on the enhancing grating it is either L – M or S. The depth

illusion is generally depending on the color for the human eye as observed from the analysis

above. The blue yellow capacity of the grating to get the desired depth is not affected by the

0

0.5

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1.5

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Mean Disparity Setting

Drifting

Static

blue color falling orientation either in the dark or in the bright part of the observed shading

during the interference. The results indicates that for the out of phase condition the blue

yellow grating obtained while doing experiment, the blue yellow grating seem to be less

affective depth suppressors in respect to the blue yellow grating when they are in phase. The

color grating that have been defined along and between the cardinal directions of the obtained

color space, shows that its results seem to match with the general results obtained by

Kingdom (2003) as for the new color directions.

The obtained cardinal directions for the human visual perceptions do not seem to be different

in case of the capacity that enhance or suppress the desired depth that leads to the final result

of the study done for the human visual perceptions. The color shading effect in the study is

moreover dependent on the contrast of the color. In this study the complete gamut of color

directions are not studied therefore it can be said that the possibility of the color directions

can be rule out for the depth enhancement. It can be observed from the results that the two

orthogonal luminance which are in orientation have the contrast and equal nature the desired

depth is less than the actual depth. The luminance gratings which have the low contrast

behave like as potent depth enhancers for the higher contrast orthogonal in orientation

ratings. The results explain that the grating effect or interference effect in deciding the depth

illusion for the human visual perceptions depends on the color contrast because it made the

difference in the depth. The results are as per the expectation and positive to the data input for

the analysis.

Relation of discussion to the hypothesis

In this study the hypothesis has the strong relation with the discussion as the discussion is

based on the analysis and the analysis is based on the obtained result where using the

approximation to the observations. The hypothesis that is used for analysing the depth of the

color directions is that the magnitude of the desired depth would be lower in case when the

depth - enhancing and depth suppressing color directions seem to be similar in respect to the

color directions looks like different. The discussion also describes that the color depth for the

human vision perceptions is completely based on the color and grating that are obtained

during the interference. L – M and S grating seem to be similar based on the capacity of

depth - enhancing, this suggests that the color depth is important. The suggestion was for the

visual system that it might be possible as plaid components that have the similar color

composition in to a single surface, and interpret that for any residual luminance the

components should be shading.

There is no much evidence available for this hypothesis therefore; there is no evidence or

support present for the color – binding idea during the experiment. Another hypothesis in

this study is that the blue yellow chromatic gratings would not be that much effective for the

suppressors’ depth because in the chromatic grating the blue phase of the grating fell in the

dark instead of felling in the bright part of the shading grating. The shaded regions on the

ground will be bluer than the non-shaded regions for the same grating of blue yellow

chromatic grating. This hypothesis is not either have much evidence available for support of

this statement. The discussion implies that even if there is no evidence available the obtained

result from the analysis through the data describes that the hypothesis somewhat matches

with the result obtained. The hypothesis explain that for the blue yellow grating which are in

out of phase the desired depth suppression will be less in respect to the blue yellow grating in

phase. In this discussion it has been found that there should not be any depth suppression if

the blue yellow gratings are in out of phase but it is there because of the relative phase’s

shifts and so this happens. The sinusoidal modulations of blue yellow gratings shifts so

because of that the color and luminance do not produce in every subject a categorical shift

form aligned to non - aligned. The another hypothesis is that there might be a possibility of

depth – enhancing for L –M grating that has been used for the blue yellow experiment in

order to have the ceiling effect to get the desired depth. Thus it can be said that the hypothesis

made during the experiment or analysis was quite good and matches with the obtained result

as discussed above.

Implications of the results

The obtained results in the study are much significant to explain that colour causes a depth

illusion in human visual perception. Color always makes illusion to the human visual

perceptions as discussed in this study that blue color and yellow color fell in the different

regions because of the grating and the depth of the color shade. In this study we find that the

all hypothesis and the conditions for the interference and grating that the depth suppressions,

was higher or can say the depth enhancement was lower in case of 90 degree than in

compared to 270 degrees grating on the ground for the depth of color share. The effect as

seen from the obtained result for both the case as in 90 degree or in 270 degree don’t have

much significant effect. There are some no parametric and parametric statistical results that

might be significant in assessing the statement for the human vision perceptions. The

parametric tests in this study are more significant and powerful then the non – parametric and

there are less chances of having the error on the side instead of getting the number of errors

on the non-parametric side. Using the errors while testing as parametric will provide the more

enhance view for the cautions that can be consider improving the human vision perceptions.

It can be seen that the obtained results in figure -7 that shows the Mean disparity threshold

for drifting and static stimuli explains that in case of drifting the setting level was higher than

the static. The present study for the Colour causes a depth illusion in human visual perception

describes based on the number of experiments and detailed analysis as done in the study that

color sensitive and luminance sensitive mechanism can be observed from it. It has been seen

that the obtained results from the study will make a good sense to the visual system to

suppress luminance boarders that will make a favour to the chromatic ones because chromatic

boarders as used in this study show the more reliable indications to the surface of the

boundaries. The results of the present study add an important caveat to this idea, by showing

that there are circumstances in which color contrast promotes luminance contrast for visual

form judgments. Given that shadows and shading can be used by the visual system for object

recognition, shape perception and motion perception, it would make sense for the visual

system to recruit color vision to help differentiate those luminance variations that are due to

shadows and shading from those that are due to changes in surface reflectance. The

suppression of shape-from-shading by aligned chromatic variations is just the other side of

the coin; the visual system makes the reasonable assumption that such luminance variations

most likely originate from changes in surface reflectance. So the implications of the obtained

result from the experiment about the colour causes a depth illusion in human visual

perception are related to the statement and signifies the importance of the color in the illusion

of depth for the human visual perceptions.

Strengths and limitations of the experiment

We replicated Kingdom’s 2003 results and got the same results for chromatic and achromatic

gratings. We got non-significant results for the phase. But this result was nearly significant.

Perhaps if we did more subjects it would be better. This shows that the illusion of the depth

for the visual perceptions for the human is right as because of the color. The intensity and

rays of the different colors are different which might result in the different depths that people

observe. Tested a hypothesis that you could get the same effect with achromatic grating in the

experiment number six but we could reject this hypothesis. This effect >does< seem to

depend on color. Some subject had noisy results -perhaps because the experiments took too

long. We could reduce the time for the experiments - perhaps by automating it more or

breaking the experiments into shorter blocks. Warn people not to wear contact lenses - their

eyes get dry. Yes the people should not go for these contact lenses as it causes problem

because it resists the moisture to the eyes so create issue. The color also gets problem in these

contact lenses and the actual thing look differently. The findings of this study are relevant to

the vision perceptions so completely argumentative and useful. The only issue is that this

result can’t be observe as 100 percent perfect because the number of experiments and

iterations are not so long as because of that the actual trend can’t be observe.

It can said that the future generation vision can be improved based on the obtained results but

as observed the things are based on the hypothesis so it cannot be assumed that this study is

100 percent perfect. There are some limitations of the study as this can’t be useful for the

accurate result but yes the trend can be observed easily. For getting the exact pattern and

perfect results there is need to have the number of consistent experiments or simulations that

will give the accurate and reliable result in comparison to this experiment. These have

generally been restricted to an artificial world where the only luminance variations present

are those arising from shading. Such models will tend to fail with more naturalistic scenes

where luminance changes due to surface reflectance are confounded with those due to

inhomogeneous illumination. These models might be made more successful if they included a

stage in which color contrasts were detected and used as local weighting functions to

strengthen uncorrelated luminance inputs (and weaken correlated inputs) to the shape-

analysis stage.

Appropriate future directions for research

For future the study is very helpful in deciding the future vision issues and the illusion that

the human have. Further depth enhancement can be made based on the number of given data

and analysis. For getting the accurate result and perfection there should be used the number

of experiments and analysis or if possible should go for the real analysis through surveys,

testing and simulations. The further research area can be done for shade effects using the

mono-chromatic gratings and should use more colors for testing.

Conclusion This study is very helpful in deciding about the human vision perceptions for the depth

illusions based on the color grating. The people should be careful for their vision and not to

sue the contact lenses to avoid any problem to the eyes. The conclusion of this study is that

Colour causes a depth illusion in human visual perception.

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